CHARACTERISTICS OF FAULTS AND SHEAR ZONES AS SEEN IN MINES AT DEPTHS AS MUCH AS 2.5~~BELO W THE SURFACE bY Robert E. Wallace and Hal T. Morris U.S. Geological Survey 345 Middlefield Road Menlo Park, CA. 94025 Sumnary The characteristics of fault and shear zones to depths of 2.5km are well documented in deep mines in North America. The characteristics can be sumrized as follows: * Fault zones generally are irregular, commonly branching and anasto- mosing rather than simple and planar. * Faults are generally constitued of one or more clay-like gouge zones in a matrix of sheared and foliated rock bordered by highly fractured rock. * The width of fault zones tend to be greater on faults having greater displacement. Fault zone on which kilometers of displacement have occurred tend to be one hundred or more meters wide; whereas those on which only a few hundreds of meters displacement has occurred are only a few meters or less wide. * Some zones represent shear distributed across hundreds of meters rather than concentrated in a narrow zone. * No striking difference is documented for the characteristics of faults over the vertical range of 2.5km. * Fault zones are invariably wet below the water table, and water moves along them at various rates, but fault zones may also serve as dams, ponding ground water several hundred meters higher on one side then on the other. 79 3 Faults and shear zones in the Coeur d'Alene mining district in Idaho. The Coeur d'Alene mining district in northern Idaho, lies in an intensely faulted and sheared structural knot (fig. l),an important structural element of which is the Osburn fault. The Osburn fault has 26km or right- lateral strike slip (Wallace and others, 1960, Hobbs and others, 19651, large enough to be similar to displacement on the San Andreas fault in California. In the process of recovering billions of dollars worth of ore from the district, the region has been cut by mine galleries, adits and shafts totalling several tens of km in length. Furthermore, ore has been mined over a vertical range of mre than 2.2km so that faults and shear zones have also been exposed over that vertical extent. At few places in the world is there a set of faults as well exposed both in lateral and vertical extent as in the Coeur d'Alcne and the similarity of the Osburn fault to the San Andreas fault warrants special attention. All types of faults are grouped and discussed together in this paper. No noteworthy characteristics that are different on normal, reverse or strike- slip faults or on faults of different ages seem obvious from the present analysis. At present, faults are exposed at the surface at elevations above 2000 meters to depths more than 300 meters below sea level. How deep the faults were buried at their time of formation is unknown. but Fryklund (19641, from a study of the environment of ore formation, ,suggests that 5km may be a likely depth for the deepest ore bodies. Inasmuch as some of the faults are exposed more than 2 km vertically below a regional up- land erosion surface, it seems reasonable to assume that some faults represent faulting at least that deep. Country rocks involved in the faulting of the Coeur d'Alene district described here are of Precambrian age and range from fine grained argillites and riltites (quartzites having silt-size grains) to fairly coarse grained quartzites. Low grade metamorphism is comn. Beds are from a few cm to meters thick, formations are hundreds to thousands of meters thick, and the total sedimentary pile exposed is more than 15km thick. The beds have been warped into folds having amplitudes of several km, so that from the highest unit on the crest of the largest anticline to the lowest unit in the trough of the largest syncline could represent several times 15km. Whether or not any of the fault characteristics described typify characteristics of faults at depths of 30 or mre km is questionable, but itsseems possible. 81 4 Continuity of faults in depth and laterally Faults in the Coeur d'Alene district characteristically are complex both in their continuity laterally and in depth. A pattern of branching and anastomosing shears describes fault patterns both in vertical section and in map plan, and at scales ranging from faults kilometers long to faultlets and shears a few meters or tens of meters long. Faults are seldom planar. Larger faults, such as the Dsburn, Placer Creek faults and branching faults such as the Alhambra and Polaris form a complex zone of shearing from 4 to 6km wide (figure 1). Figure 2 shows a complex pattern of faults and shears in cross section in the Coeur d'Alene mine, over a depth range of mre than a kilometer. Where mine workings intersect the fault zone at 200 foot intervals vertically, the pattern is demon- strated to be complex. To the north and south lie the Polaris and Saint Elmo faults, known near this cross section only at the surface and in one or two crosscuts at depth. In such situations where good exposures are absent, the complexities can not be demonstrated and faults are interpreted diagramtically as relatively uncomplicated planes. Because of such large vertical and horizontal dimensions and complexities in both fault and vein zones, exploration in the Coeur I d'Alene district has depended in large measure on extensive shaft sinking, crosscutting, and drifting. Drilling and coreing are helpful in exploration to distances of a relatively few hundred meters, but incomplete recovery of core and complexities of the fault and vein zones where cut have lead to the practice of obtaining the larger sample and mre complete record afforded by mine drifts and crosscuts. The complexities of fault zones in detail are illustrated on mine maps at original scales of 1 inch to 50 feet and larger. A detail from the Coeur d'Alene mines is shown in figure 3. Note especially the differences in the fault at points A, B, C, and D. The fault changes in strike and width over short distances in number of strands, and in the dip of individual strands of the fault in the same exposure of the fault zone. Projection of such a fault to mre than a few hundred meters, thus is difficult. In sumnary, at all scales individual faults are complex and undulating, not planar, and at all scales a network of fractures is characteristic rather than a single rupture surface. Many strike-slip faults are in reality tear faults in the upper plates of low dipping thrusts. It 1s comnly assumed that such faults term- inate at the thrust plane at a steep angle; in many areas this is not the case. In the Burgin mine of the East Tintic mining district, Utah, the ore-bearing Eureka standard tear fault strikes N. 45" E. and dips about 55" NU. from the surface to the 1,400-foot level of the Eureka Standard mine, where it has about 762 m (2,500 ft) of right lateral, horizontal displacement. As this fault is followed eastward toward the concealed trace of the north-striking, west-dipping East Tintic thrust the dip gradually decreases and the strike gradually turns northward until it merges imperceptibly with the thrust (fig. 4). Other tear faults in the Burgin mine also merge into the thrust in the same manner, indicating different amounts of tectonic transport on different parts of the thrust plate. 82 5 Geometry of fault and shear zones Few faults are truly planar features. One of the best documented deep- breaking, undulating, strike-slip faults is the Lakeshore fault of the Kirkland Lake gold-mining district in Ontario, Canada (fig. 5). This district contains the deepest mines in North America--the Lake Shore and Wright-Hargreaves mines--having reached depths of 2,461 m (8,075 ft) and 2,491 m (8,172 ) feet respectively (Charlewood, 1964). In these mines the N012'-25' E.-trending Lakeshore fault has been followed to a depth of 2,332 m (7,650 ft) and according to Hopkins (1948), shows no evidence of termination, or even of diminished throw. From the surface to the 2,300-foot level of the Wright-Hargreaves and adjacent mines the fault dips east-southeast at about 80"; at this level the dip steepens and is more or less vertical to the 3,600-foot level, where the fault becomes somewhat sinuous, but rolls over and assumes a dip of about 80" west- northwest; in the lowest levels of the mine the fault is actually north west of its surface position. Horizontal displacement on the Lakeshore fault is about 198 m (650 ft) and vertical displacement is about 99 m (325 ft). The east side has moved generally north, and north plunging striae occur on the fault plane. The fault zone is sharply defined but contains only 1-5 cm of clayey gouge. Even greater vertical irregularities are shown by the Beck and Centennial faults of the main Tintic mining district, Utah (figs. 6 and 7). Both of these faults are exposed in mine workings to depths of 825 and 550 m where they cut the vertical west limb and nearly flat trough of the Tintic syncline. These faults both strike northeasterly; the dip of the Beck fault ranges from 60' NU. to 70" SE.. and of the Centennial from 37" SE. to 70" NW. The displacement on both faults is left lateral, and is about 550 m on the Beck and 335 m on the Centennial.